1. Field of the Invention
The present invention relates to a method of and an apparatus for operating a circulating fluidized bed reactor having a separator for separating entrained solid particles from the exhaust gas and recycling the separated particles to the combustion chamber. The invention particularly relates to the composition of the bed material, and seeks to solve problems relating to the control of bed inventory and bottom ash quantity.
2. Description of the Related Art
Circulating fluidized bed reactors have been used for decades and are known to those skilled in the art of power generation, for instance. The circulating fluidized bed reactors may be gasifiers, combustors, steam generators, or any other similar type of apparatus as those skilled in the art will recognize. The fluidized bed reactors normally have an upright furnace, or reaction chamber, to the lower part of which the fuel is introduced. Primary and secondary gases, usually air, are supplied through the bottom and the sidewalls of the furnace. The combustion of the fuel takes place in a fast fluidized bed, which , in addition to fuel particles, usually also contains limestone.
A particle separator is in communication with the upper end of the furnace through a discharge port. A discharge duct connects the discharge port with the particle separator. Hot exhaust gas is discharged from the furnace, and flows through the discharge port and the discharge duct into the particle separator. The particle separator of a circulating fluidized bed boiler is usually a cyclone. When using a cyclone, the discharge duct transmits the exhaust gas with entrained solid particles tangentially into the upper portion of the cyclone separator. The cyclone separator, or other particle separator, for example, an impact separator, separates solid particles from the hot exhaust gas, which solid particles are gravity-fed to the lower end of the separator.
The lower end of the particle separator, wherein the solid particles are collected, is connected to the upper end of a vertical return duct. The opposite or lower end of the return duct has an outlet connected to the furnace for returning the separated solid particles from the particle separator to the furnace. Solid particles removed from the bottom of the furnace are referred to as bottom ash, whereas the portion of the solid material leaving the particle separator with the exhaust gases is called fly ash.
Circulating fluidized bed boilers having particle separators, for example, of the cyclone type, for separating entrained solid particles from exhaust gas and recycling the separated particles back to the combustion chamber of the boiler are well known. Examples of such systems are set forth in U.S. Pat. Nos. 4,733,621 and 5,281,398. In the former patent, the particles separated in the cyclone separator are recycled to the boiler through a split loop seal. U.S. Pat. No. 5,281,398 discloses a centrifugal separator made of flat watertube panels. This type of cyclone can be integrated with the furnace so that there is no discharge duct between the furnace and the cyclone.
The cyclone separators for circulating fluidized bed reactors have been improved over the past decades so that they have become very efficient. In normal running conditions, they may separate about 99.9 percent of the solid material leaving the combustion chamber with the exhaust gas. An efficient separation of solid particles from exhaust gases is always a property worth striving for. For instance, the better efficiency the separation has, the higher is the combustion efficiency. However, very high separation efficiency may also bring about some problems or drawbacks in the process. For instance, it may lead to a high bottom ash content compared to fly ash. When the proportion of bottom ash is high, an efficient bottom ash removal is required to maintain the required bed inventory (i.e., the composition of the bed material) in proper condition.
Since the temperature of the bottom ash is on the order of about 600 to about 900° C., ash coolers are needed to bring the ash temperature down to about 300° C., so that the ash may be safely discharged from the reactor. The more bottom ash that has to be removed from the reactor, the more expensive (i.e., of higher capacity) the equipment for both the discharge and the cooling of the bottom ash that is required.
Very high separation efficiency may also become a problem, e.g., when the quality of fuel varies in a way leading to the formation of an excessive amount of fine particles in the bed. If the particle separator recirculates a very high proportion of the fine particles, the resulting high fine particle bed inventory may lead, e.g., to a very high heat transfer efficiency in the furnace. If the heat transfer rate exceeds its designed value, the bed has a tendency to cool to a lower temperature, leading, e.g., to increased emissions to the environment.